As an improvement over the spray painting of articles such as automobile bodies and household appliances, a new kind of sheet material has been developed to provide protective and decorative coatings. The new material comprises a flexible, stretchable, thermoplastic support sheet, also known as a carrier film, which has a protective and decorative paint layer, also called a basecoat, of uniform thickness and appearance on one side and, covering the basecoat, a transparent topcoat or, as it is commonly called, a clearcoat. It can also have other layers such as a tie or bonding layer between the paint layer and the carrier film. Using known procedures such as vacuum thermoforming and in-mold bonding, the sheet material can be stretched and bonded to an article such as an automobile body panel to provide a basecoat-clearcoat finish. To facilitate the bonding of the sheet material to substrates, an adhesive layer, preferably a pressure-sensitive adhesive layer, can be formed on the back side of the carrier film. Important advantages of the paint-coated sheet material over spray painting include economy in the use of paint and avoidance of air pollution by evaporating solvents.
A new sheet material of this kind and a process for its manufacture are described in the U.S. patent application of G. G. Reafler, Ser. No. 116,426, filed Nov. 3, 1987. The process involves providing a laminar flow of the coating composition on the surface of the thermoplastic carrier film to form a layer of substantially uniform thickness, followed by a drying procedure, then coating and drying each additional layer in sequence to obtain a finished product of excellent gloss and smoothness. Paint-coated sheet material made by this process has a remarkably more attractive appearance than spray painted finishes.
The transparent topcoat, which can be clear or lightly colored, provides a number of desired properties for the paint-coated sheet material. It improves resistance to abrasion and weathering. It can also provide or improve the surface qualities of gloss and distinctness of image (DOI), as well as desired friction control properties such as slip or lubricity. Since the new sheet materials are especially intended for use as automotive coverings which demand high gloss and other surface qualities, further improvement in these qualities is desirable. Especially it is desirable to reduce or eliminate any tendency toward loss of gloss and DOI when the sheet material is stretched during thermoforming.
In the manufacture of the Reafler sheet material the transparent topcoat is applied by coating a liquid clearcoat composition onto the paint layer. In a more recent procedure described in the copending patent application of James R. Schuler, U.S. Ser. No. 313,269 filed Feb. 17, 1989 entitled "Thermoformable Sheet Material", a prefabricated transparent polymeric sheet is bonded to the paint layer before the sheet material is thermoformed. U.S. Pat. No. 4,769,100, issued Sept. 6, 1988, and European patent application No. 0,251,546, published Jan. 7, 1988, disclose a method in which a thin layer of liquid clearcoat is applied to a prepainted carrier film which, in a previous step, has been vacuum formed onto a substrate. However, neither disclosure suggests applying the clearcoat in the form of a prefabricated sheet.
The present invention offers an alternative method for applying a clearcoat to a paint-coated film or sheet. The new method provides advantages when the paint-coated sheet must be stretched substantially during thermoforming. A problem that occurs when a thermoplastic sheet having a paint coating and a clearcoat is thermoformed and stretched to a high degree is that the film loses gloss and distinctness of image in its most highly stretched areas. This problem is particularly troublesome when the paint layer contains light-reflective flakes such as aluminum flakes. Several techniques for alleviating the problem have recently been proposed. The present invention offers another way that can be used alone or with other techniques.